The research proposed is an in-depth study of some antifolate drugs and the dihydrofolate reductase/NADPH/methotrexate ternary complex using the methods of single-crystal X-ray crystallography. The specific objectives are to provide detailed and unequivocal structural information on the enzyme/cofactor/inhibitor interactions and to determine the self-associative modes and molecular conformations of the antimetabolites. Comparison of these results (using the enzyme from a normal mammal) with those forthcoming from workers on the enzyme from two mutant bacteria and that from a mammalian tumor will aid in designing drugs for better effectiveness and selectivity. The great importance of these antifolate drugs as antineoplastics, antimalarials, antibacterials, etc. will be thereby enhanced.